Enthalpy of Reaction and Hess's Law

Categories: Law Specific Heat

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I. Purpose

The purpose of this lab is to verify Hess’s Law through the three reactions of NaOH and HCl, NH4Cl and NaOH, and NH3 and HCl. The sum of the enthalpies of the first two reactions should equal the enthalpy of the third reaction.

II. Background

Hess’s Law is used to determine the enthalpy of a reaction from adding two or more preceding reactions. To determine the enthalpies of certain chemical reactions, the change in must be measured.

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This is best done using a calorimeter to prevent heat loss to the surroundings. To calculate the heat change in solution, the q=mct equation must be used. A positive value for “q” means the solution gains heat, a negative value means the solution loses heat and is exothermic. The reactions used in the lab are exothermic acid-base neutralizations. By calculating “q”, heat, the enthalpy of reaction can be found by knowing the Molarity of the reactants. The specific heat of the calorimeter can be found by the equation qcal= t x heat capacity.

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Since a calorimeter is used the heat released from the reaction will be absorbed in the solution, while some heat is transferred to the calorimeter.

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So the “q” of the reaction is given by this equation: qrxn= - (qsol+qcal).

III. Summary of Procedure

Part 1:

A calorimeter is to be arranged using Styrofoam cups and a hole on top of a cover to take the temperature while also preventing heat loss. 50mls of distilled is to be added to the calorimeter. 75mls must then be heated to 70 degrees Celsius and 50mls of that water is to be added to the calorimeter. The calorimeter has to be covered and then the temperature taken every 20 seconds for 3 minutes.

Part 2:

50mls of 2.0M HCl must be put into the calorimeter. A 50mls solution of 2.0M NaOH should then be added to the HCl solution and stirred. The temperature needs to be recorded every 20 seconds for 3 minutes. For the second reaction the same process has to be repeated using 2.0M NH4Cl and 2.0M NaOH. The third reaction requires the same process using 2.0M solutions of NH3 and HCl.

IV. Observations

* Styrofoam cup becoming hot after mixture
* thermometer scraping the Styrofoam cup sides
* cover doesn’t completely cover the cup
* small amount of time between pouring the chemicals and covering and mixing

VI. Results and Questions

Calculations please see attached graphs and work.
Post Lab Questions

1. What is meant by calorimetry?

Calorimetry is the scientific measuring of heat released during chemical and physical changes. It ensures that minimal heat is lost so the heat of reaction can be found accurately.

2. How does graphical analysis improve the accuracy of the data?

Once the points are plotted on a graph the line of best fit can be drawn and extrapolated toward the y-axis. Since the first data plots can be inconsistent the best fit line may ignore the first points making the data more accurate by disregarding the inaccurate data.

3. What is the meaning of the negative sign in front of the equation for heat of reaction?

The negative sign in front of the brackets indicate that we are in fact solving for the reverse reaction. So the sign must have a negative to indicate the reaction that is the forward, exothermic, reaction.

4. Do the lab results support Hess’s Law?

The lab does support Hess’s Law. The percent error is relatively small for this lab and algebraically it is proven that the sum of the enthalpies of the first two reactions is close the measured value of the third reaction.

5. How could the procedure be modified to achieve greater accuracy?

To improve the accuracy of the lab a formal capacitor could be used instead of average Styrofoam cups. An airtight seal between the cover and thermometer would also be more accurate by preventing heat loss to the surroundings.

6. Find a table reference that lists standard heat of formation for the species included in your net ionic equations. Use them to calculate delta H for the reaction of the net ionic equations. Do these values support Hess’s Law?

See attached calculations.

VII. Conclusion

This lab successfully verified Hess’s Law by adding enthalpies of reactions to equal the enthalpy of a third reaction. The mix of NaOH with HCl, and NH4 with NaOH proved to give off a heat close to that of NH3 mixed with HCl. The percent error proved to be 3.72%. The sources of error could be the heat lost while the calorimeter was not covered and maybe not enough mixing throughout the duration of the three minutes. The addition of a mixing unit and an airtight seal on the capacitors would provide an improved version of this lab.